This section provides background information related to the present disclosure which is not necessarily prior art.
Power transfer assemblies of the type used in motor vehicles such as, for example, four-wheel drive (4WD) transfer cases, all-wheel drive (AWD) power take-off units and axle drive modules are commonly equipped with a torque transfer mechanism. Such torque transfer mechanisms are configured and operable to regulate the transfer of drive torque from a rotary input member to a rotary output member. Typically, the torque transfer mechanism includes a multi-plate friction clutch operably disposed between the input and output members and a clutch actuator for engaging the friction clutch. The degree of clutch engagement, and therefore the amount of drive torque transferred, is a function of the clutch engagement force applied to the friction clutch via the clutch actuator. The rotary input and output members may include a pair of shaft, as is conventional in transfer cases, or a gearset such as is provided in power take-off units.
The clutch actuator typically includes a drive mechanism and a clutch operator mechanism. The clutch operator mechanism is operable to convert the force or torque generated by the drive mechanism into the clutch engagement force which, in turn, is applied to a multi-plate clutch pack associated with the friction clutch. The drive mechanism can be passively actuated or, in the alternative, can include a power-operated device which is controlled in response to control signals from an electronic control unit (ECU) associated with a traction control system. Variable control of the control signals is typically based on changes in road conditions and/or the current operating characteristics of the vehicle (i.e., vehicle speed, acceleration, brake status, steering angle, interaxle speed differences, etc.) as detected by various sensors associated with the traction control system. As such, highly precise control of the drive torque transferred in such adaptive or “on-demand” torque transfer mechanisms permits optimized torque distribution during all types of driving and road conditions.
One factor that impacts the precision or accuracy of the drive torque actually transferred across the friction clutch is the frictional interface between the interleaved clutch plates associated with the multi-plate clutch pack. When the clutch pack is partially engaged, the clutch plates slip relative to one another and generate heat. As is known, lubricating fluid may be routed from a lubricant sump within the power transfer assembly to flow through and around the clutch pack to cool the clutch plates as well as the other clutch components in addition to lubricating bearings and other rotary components. It is well documented that excessive heat generation can degrade the lubricating fluid and damage the clutch plates. It is also well documented that excessive pressure within the power transfer assembly generated via driven rotation of the components of the torque transfer mechanism can detrimentally impact the service life of bearings and seals.
A number of different types of lubrication systems are used in current power transfer assemblies. One lubrication system employs a shaft-drive fluid pump (i.e., gerotor pump) that functions to generate a pumping action for supplying lubricating fluid from the lubricant sump to the friction clutch in response to rotation of a driven shaft. Such shaft-driven fluid pump lubrication systems are typically inefficient due to the continuous pumping operation and the large pumping capacity required to provide adequate lubricant flow rates at both low and high rotational speeds. Another type of lubrication system used in some power transfer assemblies, referred to as a “pump-less” system, relies on the rotary components to transmit the lubricating oil from the lubricant sump to the friction clutch. While such systems are capable of eliminating the need for a pump to provide the lubricant flow requirements, the flow rate and capacity is still directly proportional to the rotary speed of the lubricant-carrying components. It is also known to provide a shaft-driven fluid pump with a pump clutch that is operable for selectively coupling and uncoupling a pump component to the shaft to provide a “disconnectable” pump assembly. Such an arrangement permits on-demand operation of the fluid pump, but its flow rate and capacity are still a function of the shaft speed. Finally, it is also known to mount an electric fluid pump within the lubricant sump. The submerged electric pump can provide on-demand pumping operation independent of shaft speed.
In view of the above, virtually all power transfer assemblies of the type used in motor vehicle drivetrain and driveline applications have components that rotate at very high speeds in an enclosed chamber formed within the housing and which are bathed in, or supplied with, lubricating fluid from the lubricant sump for lubrication and cooling purposes. Due to these high rotational velocities, pressure and heat tend to build up within the enclosed chamber. As such, many power transfer assemblies are also equipped with a venting system having a valve configured for allowing the venting of pressurized air from the enclosed chamber to ambient so as to ensure longer life of the gaskets and seals. In many power transfer assemblies, the valve associated with the venting systems also permit ambient make-up air to be drawn back into the enclosed chamber, without introduction of contaminants, upon subsequent cooling of the lubricating fluid. However, one known drawback associated with conventional venting arrangements is the unintended escape of some lubricating fluid and/or failure of the vent valve due to clogging thereof with contaminants or lubricating fluid.
In some power transfer assemblies, a long vent tube hose is connected between a vent tube fitting on the housing and a remote location within the engine compartment to reduce the risk of contamination ingress and oil burping or leaks, as an option to direct venting systems having a vent valve mounted directly to the housing of the power transfer assembly.
In view of the above, it is recognized that optimized venting and lubricant containment arrangements are required for use with power transfer assemblies to provide enhanced venting and pressure stabilization for extending the service life of the sealing components as well as for preventing escape of lubricating fluid and deterioration of the venting components. Thus, a need exists to develop improved lubrication and venting systems for use in power transfer assemblies which overcome the shortcomings of conventional venting arrangements.